Method of retouching etchings in intaglio printing forms and printing forms treated according to this method



te tates WTHDD F RETOUCHING ETCHINGS IN INTAGLIO PRINTING FORMS AND PRINT- ING FURMS TREATED ACCORDING TO TEES METHOD Lambertus H. Bosman, Haarlem, Netherlands, assignor to N.V. Quod Bonum, Haarlem, Netherlands, a corporation of the Netherlands This invention relates to the retouching of etchings in intaglio printing forms. In actual practice these are substantially always cylinders and for conveniences sake the specification will therefore refer to intaglio printing cylinders.

If proofs are taken from intaglio printing cylinders prepared for printing, it often appears that in certain areas the etching cavities are too shallow or too deep.

In the case of non-chromium plated cylinders, for example copper cylinders, an area in which the etching cavities are too shallow may be subjected to a second etching process by applying a stiff paint to the lands (i.e. the portions of the cylinder surface that have not been affected by the etching treatment) in the area to be retouched and subsequently etching said area with a solution of iron chloride so as to make the etching cavities deeper. It is not easy to correctly perform this method because the second etching process is liable to deepen the etching cavities too much and to damage the lands. Such a retouching cannot be undone any longer.

If it is desired to correct too deep an etching this is possible by galvanically precipitating copper on the entire area to be corrected and subsequently lowering the lands again by a polishing operation. This, however, entails the risk that the lands will be damaged, especially adjacent the deepest etching cavities.

In the case of chromium-plated cylinders it has been impossible hitherto to correct the etching without completely removing the chromium from the cylinder.

In the case of copper cylinders a Wrong etching or a damaged area may be made plain again by filling the etching cavities or the damaged area with solder or a thick liquid paste which will harden after being dried. However, there is always the risk that an area so corrected will cause a fogging on the paper during the printing.

In the case of chromium-plated cylinders the entire filling up a wrong etching or of a damaged area has hitherto been practically impossible owing to the poor adherence of the filling mass to the chromium, so that during the printing operation the filling mass was entirely or partly loosened.

This invention relates to a method of retouching etchings in intaglio printing cylinders by changing the depth of the etching cavities in the area to be retouched, which method is characterized in that this is effected either by applying an insulating layer to the lands and changing the depth of the etching cavities by galvanic etching or by precipitating metal therein, or in the alternative by applying an insulating layer to the etching cavities and changing the height of the lands by precipitating metal thereon or by etching the lands galvanically or chemically, whereupon the insulating layer is removed again.

In many cases it is of advantage to apply one or more auxiliary metal layers to the area of the form to be retouched prior to changing the height of the lands or the depth of the etching cavities, said auxiliary metal layers Patented Aug. 23, 1960 ice may have a different function. They may serve as an intermediate layer promoting the adherence of the metal to be precipitated on the lands or in the etching cavities or of a second layer of metal, which is to cover the entire area to be retouched, to the base metal. It is also possible to apply a layer of metal which will dissolve in an etching agent which will not attack the base metal. Furthermore it is possible to apply two layers of metal of which the upper layer will dissolve in the etching agent to be used, whereas the lower will not be attacked. In this case an etching agent may be used in which the base metal will dissolve, because said base metal will be protected by the lower layer, the so called etching retarding layer, from the action of the etching agent.

Some applications of the principle explained hereinbefore will now be described.

According to a simple embodiment of the invention the etching may be deepened by galvanically etching the etching cavities in the area to be retouched to a greater depth after insulating the lands. This method, for example, very well lends itself to be applied to copper intaglio printing cylinders. In this connection the metal of the intaglio printing cylinder is the metal of which the surface of said cylinder consists, also if said metal only constitutes the coating of a cylinder consisting of another metal, because the latter metal does not play a part in the printing operation.

Conversely the etching may be made less deep by insulating the etching cavities and by removing the upper portion of the lands by a galvanic or chemical etching treatment.

Another simple correction method consists in that after insulating the etching cavities metal is precipitated on the lands or after insulating the lands metal is precipitated in the etching cavities. In the first instance the etching is deepened, in the second case it is rendered shallower. This method, for example, is suitable for brass cylinders on which copper may be galvanically precipitated without any difliculty.

For insulating the etching cavities an acid resistant paste may be successfully used, which paste may consist for example of a mixture of a wax and an asphalt lacquer. The etching cavities in the area to be retouched are filled with this paste, the lands remaining free.

For insulating the etching cavities it is also possible to use a metal, for example nickel, which is applied to the entire area to be retouched and which is subsequently removed from the lands by polishing, so that on said lands the original metal surface is laid bare again. The metal introduced into the etching cavities should then be such that it will not dissolve in the etching agent of the base metal.

The lands may be insulated by applying a stifi, acid resistant paint, for example book-printing or offset ink, to the lands in the area to be retouched by means of rollers, the etching cavities remaining unfilled.

As the correction is only to be efiected on a part of the cylinder, the remainder of the cylinder should be protected during the treatment of said part. In principle this may be effected by using a stencil which leaves the area to be retouched uncovered or by glueing a stencil on the area to be retouched, covering the surrounding part with an insulating lacquer, for example a mixture of an asphalt lacquer and a plasticizer, and subsequently removing the stencil again. The manner in which this may be efiected will be described in detail in the examples.

In so far as the correction is efiected galvanically this is preferably carried out by means of a small tampon serving as the anode or as the cathode. By moving said tampon by hard metal may be locally precipitated on or be locally etched away in the areas to be corrected, so

that efiects may be reachedwhich are suggestive of photographic retouches. An advantage of any galvanic retouch is that the number of ampere-seconds is indicative of the amount of metal precipitated or dissolved. The desired alteration in tonal value may be expressed therefore in the time of the treatment. i

The retouching methods described hereiubefore present the disadvantage that the correction made cannot be undone if it appears that the result obtained is unsatisfactory. This disadvantage does not present itself in the embodiments of the invention described hereinafter, in which one or a plurality of auxiliary metal layers to be applied to the entire area to be retouched are employed. The etching in this case may be efiected both galvanically and chemically.

It is possible, for example, to provide a uniform coating of zinc onto the entire area to be retouched of a copper cylinder, said coating having the same thickness on the lands and in the etching cavities; after either the lands or the etching cavities have been insulated the area thus coated is etched with an acid which dissolves the zinc but not the copper, so that in the non-insulated portions of the area to be retouched the zinc will be removed. After the removal of the insulating substance a surface is obtained in which either the height of the lands has been increased by the zinc or coating the etching cavities has been partly filled with zinc, so that, the etching has become correspondingly deeper or lower.

If such a correction is found to be unsatisfactory, the entire area to be retouched may be treated with the etching acid, so that the remaining zinc will be dissolved and the plate is again restored to its original condition.

It is possible to proceed in the same manner in the case or" a chromium-plated cylinder which is not attacked by the majority of etching agents. As, however, many metals do not adhere to chromium it is mostly desirable first to provide an intermediate coating over the entire area to be retouched and to precipitate thereon the metal proper to be etched. This last layer of metal may be for example a layer of nickel or a layer of another hard metal. After this treatment, therefore, the entire area to be retouched has been heightened by two layers of metal without the difference in height between the lands and the etching cavities being changed.

Subsequently either the etching cavities or the lands in the area to be corrected are insulated. If now the etching is carried out with a suitable etching agent in which both metals will dissolve, the two layers will be removed by the etching down to the chromium in the uncovered places, so that a corresponding lowering or heightening of the lands relative to the etching cavities will be obtained.

In this case too the correction may be entirely undone by removing the insulating substance and by etching the entire area to be retouched. The portions of the two layer of metal left behind on the insulated places will then be dissolved, so that the plate is restored to its original condition.

In an analogous manner the chromium may also be coated with an intermediate layer and layer of copper and after insulating the lands or the eching cavities the copper and the intermediate layer may be removed by etching with an iron chloride solution.

Another method consists in the application of a socalled etching retarding layer which will not dissolve in the etching agent. The etching agent will then only act on the metal provided on top of said layer, but will not attack the metal of the cylinder. 7

We may, for example in the case of a copper cylinder provide the entire area to be retouched with a layer of nickel and deposit a layer of copper on said. layer of nickel. After insulating the lands or the etching cavities the cylinder is etched with an etching agent for copper which does not attack nickel e.g., a mixtureofchromic acid and sulfuric acid. In the uncovered places only the 4 upper layer of copper will be attacked, but the nickel and the metal of the cylinder.

A retouch thus obtained also oifers the advantage that the correction may be undone by treating the entire retouched area with the etching agent. The remainder of the upper layer of copper will dissolve then, so that only the layer of nickel is left, which, however, uniformly covers the original surface and which, therefore, has no influence on the difference in height between the lands and the etching cavities.

According to another embodiment the depth of the etching cavities in chromium plate copper cylinders may be modified by a double etching treatment while using etching retarding layers. In this case the chromium on the lands or in the etching cavities is removed and subsequently the bared copper is removed.

It is possible, for example, after the provision of an insulating mass in the etching cavities to galvanically apply a nickel-cobalt layer and subsequently a copper layer to the lands. The nickel-cobalt'layer then serves as an intermediate layer to improve the adherence of the copper to the chromium. After removal of the insulating mass from the etching cavities the chromiurn'is removed from said etching cavities by etching with hydrochloric acid, the copper on the lands acting as an etching retarding layer. The plate is then etched with an etching agent for copper, for example an iron chloride solution. The

7 copper in the etching cavities will thereby be dissolved so that said cavities become deeper. At the same time the copper on the lands and also the nickel-cobalt layer will dissolve in the etching agent; the subjacent layer of chromium will then act as an etching retarding layer for the copper. The ultimate result will be that the etching cavities consisting of copper, have been made deeper whereas the lands remain covered with chromium.

It is also possible to proceed in an analogous manner, while insulating the etching cavities instead of the lands.

. p The ultimate result will be a reduction in height of the lands which consist of copper, whereas the etching cavities remain coated with chromium.

A similar result may be obtained by applying to the entire area to be retouched of the chromium plated copper intaglio printing cylinder an intermediate layer consisting of nickel cobalt and subsequently a copper layer. After insulating the lands the plate is first etched with process an etching agent for copper, whereby the nickel-cobalt layer is also dissolved and subsequently with an etching agent for chromium. Thereupon the insulating mass is removed and the relative area is etched again with the etching agent for copper. The bared copper in the etch ing cavities will thereby be further etched,.while the copper layer and the nickel-cobalt layer on the lands will also dissolve. The layer of chromium laid bare by this second etching treatment, will, however, act as an etching retarding layer, so that the height of the lands remains unchanged, whereas the etching cavities become deeper.

The insulating mass may again be provided in the etching cavities instead of on the lands. The ultimate result then will be that the height of the lands is reduced relative to the bottom of the etching cavities.

Hereinbefore several methods of retouching etched cylinders having a copper or chromium surface with the application of various auxiliary metals have been described. However, it is also possible to make corrections in a similar manner in intaglio printing cylinders consisting of other metals e.g. steel and to use other metals for the auxiliary layers.

When making corrections While using auxiliary metal layers it often occurs that owing to the difference in height in the corrected and the non-corrected area of the cylinder a difference in tone will be obtained, so that a separating line between the corrected and the non-corrected area of the cylinder will be visible. This difliculty may be met by providing the auxiliary layer or layers over an area larger than the area to be corrected.

In that case a more gradual change is obtained, especially if the thickness of the auxiliary layer gradually diminishes in the direction away from the area corrected. This will be explained in detail in some of the examples.

The method according to the invention may also be employed for entirely filling up etchings and for making scratches and other defects invisible. If an etching or such a defect has been filled up a new etching may be provided on the area this treated.

if in the area corrected the upper layers of the lands and the upper layer in the etching cavities consist of difierent metals, the corrections obtained according to the invention may be intensified or weakened without the provision of an insulating layer, by subjecting the corrected area to such a treatment that only on one of the two metals a layer of metal is deposited or that only one of the two metals is removed by etching. The first method is of special importance for chromium-plated cylinders, because most metals will be e.g. copper, will not properly adhere to chromium, so that it is easy to carry out the method so as to deposit the metal only on the auxiliary metal. The second method may be used both for copper cylinders and to chromium-plated cylinders.

The invention will be elucidated with reference to the following examples.

Example I in an etched copper intaglio printing cylinder all the etching cavities both of the area to be corrected and of the surrounding area are filled with an insulating, acid resistant paste consisting of a mixture of wax and asphalt lacquer. Subsequently a stencil is glued to the area to be corrected and the surrounding area is covered with an insulating lacquer consisting of a mixture of asphalt lacquer and a plasticizer.

After removing the stencil copper is galvanically or chemically precipitated onto the uncovered portion of the cylinder whereby the lands are heightened, whereas the etching cavities remain unchanged. The insulating lacquer and the insulating paste are removed from the cylinder.

Example I] The pre-treatment of the cylinder is the same as in Example I, but instead of precipitating copper onto the lands, said lands are etched galvanically or chemically. In this manner a cylinder is obtained in which the etching has become less deep in the area retouched.

Example 111 On a copper intaglio printing cylinder the lands both in the area to be corrected and in the surrounding area are covered with an oflFset ink by means of a roller. A stencil is glued on the area to be corrected whereupon the surrounding area is covered with an acid resistant lacquer. The stencil is subsequently removed and the etching cavities in the area to be corrected are filled either galvanically or chemically with copper to such an extent that the depth of the etching is reduced to the desired extent. The insulating lacquer and paste are then removed.

Example IV The cylinder is treated in the same manner as in example III, but instead of filling the etching cavities with copper they are etched galvanically. In this way the etching is deepened.

Example V Both on the area to be corrected and on the surrounding part of a copper intaglio printing cylinder a layer of nickel is deposited galvanically. Copper is then deposited galvanically on an area larger than the area to be corrected, but smaller than the area covered by the layer or" nickel. The area to be corrected, therefore, is covered with two uniform layers of metal, but the depth of the etching is unchanged. A stencil surrounding the area to be corrected is now glued onto the cylinder and in said area the lands are insulated by means of an acid resistant paint. Said area is then etched with a chromic acid-sulphuric acid solution effected, which dissolves the copper, but not the nickel. It is possible to continue the etching down to the layer of nickel or to a lesser extent. Thereafter the mea to be corrected is entirely covered with an acid resistant lacquer, the stencil is removed and in the area surrounding the area to be corrected the copper is removed down to the etching retarding layer by means of a chromic acid-sulphuric acid solution.

After this treatment the etching cavities in the area to be corrected have become deeper relative to the lands covered with nickel and copper, while in the part surrounding the area to be corrected both the lands and the etching cavities are covered with a layer of nickel, so that the depth of the etching has not been changed there. There is now a gradual transition from the corrected to the non-corrected area.

Example VI The procedure is the same as in the preceding example, the difference being however, that in the area surrounded by the stencil not the lands, but the etching cavities are insulated. In this case the etching is made less deep by etching the copper on the lands away down to the etching retarding layer.

Example VII On a copper cylinder a layer of zinc is provided galvanically, said layer covering both the area to be corrected and the surrounding area. A stencil enclosing said area is glued onto the cylinder and said area is insulated by means of an acid resistant lacquer. After removing the stencil the cylinder is etched with a zinc etching acid, so that all of the zinc surrounding the area to be corrected is removed. The acid resistant lacquer is then removed from the area to be corrected and the etching cavities are filled with an insulating paste, whereupon the zinc on the lands is removed by etching. By this treatment the etching has become less deep.

Example Vlll On a chromium-plated intaglio printing cylinder the etching cavities both in the area to be corrected and in the surrounding part are filled with an insulating paste, whereupon a stencil is glued onto the area to be corrected. The surrounding part is subsequently insulated with lacquer whereupon the stencil is removed. A nickelcobalt layer is galvanically applied to the lands as an intermediate layer and a layer of nickel is galvanically deposited on the intermediate layer. Thereafter the insulating lacquer and the insulating paste are removed again.

Example IX A chromium-plated cylinder is provided galvanically both on the area to be corrected and on the surrounding part, with a nickel-cobalt layer as an intermediate layer and thereafter with a layer of nickel. The lands are covered with an insulating paint by means of a roller and the etching cavities are etched down to the chromium with a solution of iron chloride in water. A stencil surrounding the area to be corrected is then glued on the cylinder and said area is entirely covered with an in sulating lacquer. After the removal of the stencil the underlying part is etched down to the chromium, whereupon the insulating lacquer is removed.

Example X The area to be retouched of a chromium-plated cylinder is covered with a stencil and the surrounding area is coated with an insulating lacquer. After removal of the stencil the non-coated area is galvanically provided with an intermediate layer consisting of nickel-cobalt and a copper-layer, whereupon the etching cavities in Example X1 A similar result may be obtained by galvanically depositing the intermediate layer and the copper layer both on the area to be retouched and on the surrounding part of the'cylinder referred to in the preceding example, insulating the etching cavities with a paste and etching the copper and the intermediate layer on the lands away by means of an iron chloride solution.

I Subsequently a stencil surrounding the area to be corrected is glued onto the cylinder and said area is covered with an acid resistant lacquer. After removal of the stencil the underlying part is etched down to the chromium and finally the acid resistant lacquer is removed.

Example XII The methods described above for deepening the etching in chromium-plated cylinders only give good results if the correction to be made is slight. If the lands are heightened to a great extent, the'difiiculty is encountered thatafter printing a line ofcontrast between the corrected and the non-corrected area will be clearly visible. In this case the method may better be carried out as follows:

Both the area to be corrected and the surrounding area are galvanically heightened with an intermediate layer of nickel-cobalt and a superposed layer of nickel. The lands are then covered with an insulating paint by means of a roller and a stencil is glued onto the area to be corrected. After the surrounding area has been covered with an insulating lacquer the stencil is removed and in the area to be corrected the nickel and the intermediate layer are removed from the etching cavities by an etching treatment with an iron chloride solution.

In this case in the area corrected the etching will have become deeper by the lands being heightened. The surrounding area is coated with the two layers of metal, so that the ditference in height between the lands and the etching cavities as compared with the original etching is not changed, but the lands have the same heightas in the corrected area. When printing with a cylinder thus retouched the separating 'lineYwill be much less visible than without the treatment according to this example. i r i Example XIII On the area to be retouched of a chromium plated copper cylinder a stencil is provided and the surrounding area is covered with an insulating lacquer. After removing the stencil an insulating paste is provided in the etching cavities of the area to be retouched. On the a lands an intermediate layer consisting of nickel-cobalt and subsequently a layer of copper is galvanically deposited. After removing the insulating paste from the etching cavities the chromium in the etching cavities is removed by etching down to the copper with hydrochloric acid. The area is then etched with a solution of iro n chloride in water, whereby the etching cavities are etched deeper, while at the same time the copper layer and V the nickel-cobalt layeron the lands will dissolve. Finally the insulating lacquer is removed. As a result of the above treatment the etching has become deeper.

Example XIV cally coated with an intermediate layer consisting of nickel-cobalt and subsequently with a layer of copper. The lands are then insulated by means of an insulating taglio printing form which .of the printing form is copper,

. ing said area with an etching agent i the metal of said printing paint that is applied by means of a roller, whereupon the area is first etched with an iron chloride solution and thereafter with hydrochloricacid, so that the copper layer, the nickel-cobalt layer and the chromium layer in the etching cavities are dissolved.

After the insulating paint has been removed from the lands the etching with an iron chloride solution is repeated so that the etching cavities are etched further, whereas on the lands only the layerof copper and the layer of nickel-cobalt will be removed. Finally the in sulating paint is removed.

The etching has become I claim:

1. A method of retouching an etching in a metal inhas a screen bridge portion and an etching cell portion, comprising the steps of coating one of said portions in the area to be retouched with a non-metallic etching agent resisting'material, depositing at least one auxiliary metal layer on the other of said portions, removing said etching agent resisting material and etching said area. with an etching agent capable of dissolving the metal of the printing form and incapable of dissolving the auxiliary metal.

2. A method according to claim 1 in which the metal the auxiliary metal is nickel and the etching agent is a dilute solution of chromic acid and sulfuric acid. a

3. A method of retouching an etching in a metal intaglio printing form which has a screen bridge portion and. an etching cell portion, comprising the steps of coating one of said portions in the area to be retouched with a non-metallic etching agent resisting material, depositing at least one auxiliary metal layer on the other of said portions, removing said etching agent resisting material, etchcapable of dissolving form and incapable of dissolving the auxiliary metal, and subsequently etching said area with an etching agent capable of dissolving the auxiliary metal and incapable of dissolving the metal of said printing form.

4. A method according to claim 3 in which the metal of said printing form is chromium, and two auxiliary metal layers are provided, the first of which consists of 'an alloy of nickel and cobalt and the second layer consisting of copper, the first etching agent is dilute hydrochloric acid and the second etching agent is a dilute ferric chloride solution.

5. A method according to claim 3 in which the metal of the printing form is chromium and copper, the chromium being in the form of a thin coating on copper, two auxiliary metal layers are provided, the first of which consists of an alloy of nickel and cobalt and the second of copper, the first etching agent is a solution of hydrochloric acid and the second etching agent is a solution of ferric chloride, the treatment with the hydrochloric acid solution being continued until the chromium coating on the areas exposed to said hydrochloric solution is completely dissolved so that by the subsequent treatment with the solution of ferric chloride both the copper of the second auxiliary layer and the copper of the printing form in the surface portion where the chromium has been dissolved are attacked.

'6. A method of retouching an etching in a metal base intaglio printing form which has a screen bridge portion and an etching cell portion, comprising the steps of de positing at least one auxiliary metal layer over the whole area to be retouched, coating one of said portions with a non-metallic etching agent resisting material, changing the height of the other of said portions by etching said area with an etching agent capable of dissolving the auxiliary metal, but incapable of dissolving the base metal, and removing said etching agent resisting material.

7. A method according to claim 6 in which the base metal is copper, the auxiliary metal is zinc and the etching agent is a hydrochloric acid solution.

deeper by this treatment.

8. A method of retouching an etching in a metal base intaglio printing form which has a screen bridge portion and an etching cell portion, comprising the steps of depositing two auxiliary metal layers over the whole area to be retouched, coating one of said portions with a nonmetallic etching agent resisting material, changing the height of the other of said portions by etching said area with an etching agent capable of dissolving the upper auxiliary metal but incapable of dissolving the lower auxiliary metal, and removing said etching agent resisting material.

9. A method according to claim 8 in which the base metal is copper, two auxiliary metal layers are provided, the first of which consists of nickel and the second layer consists of copper, and the etching agent is a solution of chromic acid and sulfuric acid.

10. A method of retouching an etching in a metal base intaglio printing form which has a screen bridge portion and an etching cell portion, comprising the steps of depositing two auxiliary metal layers over the whole area to be retouched, coating one of said portions with a nonmetallic etching agent resisting material, changing the height of the other of said portions by etching said area with an etching agent capable of dissolving both auxiliary metal layers, but incapable of dissolving the base metal and removing said etching agent resisting material.

11. A method according to claim 10 in which the base metal is chromium, two auxiliary metal layers are provided, the first of which consists of a nickel-cobalt alloy and the second layer consists of copper, and the etching agent is a ferric chloride solution.

12. A method of retouching an etching in a metal base intaglio printing form which has a screen bridge portion and an etching cell portion, comprising the steps of de positing at least one auxiliary metal layer over the Whole area to be retouched, coating one of said portions with a non-metallic etching agent resisting material, changing the height of the other of said portions by etching said area with an etching agent capable of dissolving the auxiliary metal and subsequently with an etching agent capable of dissolving the base metal but incapable of dissolving the auxiliary metal, and removing said etching agent resisting material.

13. A method according to claim 12 in which the base metal is copper, the auxiliary metal is nickel, the first etching agent is a ferric chloride solution and the second etching agent is a solution of chromic acid and sulfuric acid.

14. A method of retouching an etching in a metal base intaglio printing form which has a screen bridge portion and an etching cell portion, comprising the steps of depositing at least one auxiliary metal layer over the whole area to be retouched, coating one of said portions with a non-metallic etching agent resisting material, changing the height of the other of said portions by etching said area with a first etching agent capable of dissolving the auxiliary metal but incapable of dissolving the base metal, and subsequently with a second etching agent capable of dissolving the base metal but incapable of dissolving the auxiliary metal, removing said etching resisting material and etching said area with a third etching agent capable of dissolving the auxiliary metal so as to remove the same from the first mentioned portion.

15. A method according to claim 14 in which the base metal is chromium, two auxiliary metal layers are provided, first of which consists of a nickel-cobalt alloy and the second consists of copper, the first and third etching agent is a ferric chloride solution and the second etching agent is a hydrochloric acid solution.

16. A method according to claim 14 in which the base metal is chromium, two auxiliary metal layers are provided, the first of which consists of a nickel-cobalt alloy and the second consists of copper, the first and third etching agent is a ferric chloride solution and the second etching agent is a hydrochloric acid solution, the treatment with hydrochloric acid being continued until the chromium layer on the areas exposed to said hydrochloric acid solution is completely dissolved whereby the underlying copper can be attacked by the third etching agent.

References Cited in the file of this patent UNITED STATES PATENTS 1,832,716 Lopez Nov. 17, 1931 2,083,249 Thomson June 8, 1937 2,233,546 Meulendyke Mar. 4, 1941 2,367,236 Murray Jan. 16, 1945 2,572,228 Whyzmuzis Oct. 23, 1951 2,584,317 Aller Feb. 5, 1952 2,599,914 Hartsuch et al June 10, 1952 2,602,731 Nierenberg July 8, 1952 2,607,825 Eisler Aug. 19, 1952 2,748,031 Kafig May 29, 1956 2,829,460 Golay Apr. 8, 1958 2,848,359 Talmey Aug. 19, 1958 FOREIGN PATENTS 503,210 Great Britain June 26, 1937 655,053 Great Britain July 11, 1951 

1. A METHOD OF RETOUCHING AN ETCHING IN A METAL INTAGLIO PRINTING FROM WHICH HAS A SCREEN BRIDGE PORTION AND AN ETCHING CELL PORTION, COMPRISING THE STEPS OF COATING ONE OF SAID PORTIONS IN THE AREA TO BE RETOUCHED WITH A NON-METALLIC ETCHING AGENT RESISTING MATERIAL, DEPOSITING AT LEAST ONE AUXILIARY METAL LAYER ON THE OTHER OF SAID PORTIONS, REMOVING SAID ETCHING AGENT RESISTING MATERIAL AND ETCHING SAID AREA WITH AN ETCHING AGENT CAPABLE OF DISSOLVING THE METAL OF THE PRINTING FROM AND INCAPABLE OF DISSOLVING THE AUXILIARY METAL. 